As important components in the field of optical communication, optical transmission modules having basic functions such as electro-optic conversion, photoelectric conversion, amplification, reproduction, and modulation/demodulation are used in a wide range of applications. With the background of speed-up and a capacity increase of wired networks and wireless networks, the optical transmission modules are required to perform signal processing at higher speed, and are strongly required at the same time to achieve cost reduction and enhancement of reliability. The optical transmission module achieves high speed signal transmission by using a flexible printed wiring substrate to electrically connect a substrate on which optical semiconductor elements (such as a semiconductor laser, a light receiving element, and a modulation element) are mounted, and a substrate on which signal generator circuits (such as a driver circuit and an amplifier circuit) are mounted.
The optical transmission modules which establish connection by using flexible printed wiring substrates have been advancing in downsizing of their packages, speed-up of electric signals, and an increase in the number of arrays in their internal structures. One of known examples of such latest optical transmission modules advanced in downsizing and speed-up is a receiver module of pluggable optical transceiver (ROSA: Receiver Optical Sub-Assembly) as described in Non-Patent Document 1. For example, in the latest downsized 100 Gb/s ROSA module, electric signal wires for four channels having a transmission rate of 25 Gb/s and a large number of DC bias lines are formed on a package with a width of 7.0 mm. In the case of constructing input/output wires on one side of the package, it is necessary to connect the inside of the module to an outside by the electric signal wires and the power supply wires within a range of the one side having a width of 7.0 mm, for example.
A flexible printed wiring substrate (FPC: Flexible Printed Circuit) is made of a material thinner and more flexible than that for an ordinary printed wiring substrate, and is provided with a conductor foil formed using a film-form insulator as a base, for example. A flexible printed wiring substrate is connected to a circuit substrate or the like in such a way that the electrodes at the tip end portions of the signal wiring patterns formed on the respective substrates are fixed to each other by soldering. In the following description, the flexible printed wiring substrate is simply referred to as a “flexible substrate”.
As a mounting method of fixing electronic components or the like on a substrate by soldering, a method is widely known in which chip components of a surface mount type are fixed in an almost-completely automated manufacturing process using a screen-printed cream solder and a reflow furnace. For components with special shapes or sizes, components with limited heat resistance, or the like, there is a method of mounting the components, for example, by applying a solder to the printed substrate, and then performing an instantaneous heating method using a hot bar (heater tip). In addition, there is also known a mounting method in which a solder on a substrate is preliminarily reflowed and melted, and then is re-reflowed by an instantaneous heating method (Patent Literature 1).
In the case of connecting the 100 Gb/s ROSA module described above by using a flexible substrate, it is necessary to form a large number of wires and connection terminals of the respective wires within a very narrow width of 7 mm on one side of the package of the module, and to form corresponding connection terminals on the flexible substrate. If the numbers of signal wires and power supply wires are increased, a situation may occur in which a single flexible substrate cannot connect all the wires.